The advent and subsequent development of optical communication systems which employ high transmission speeds and frequencies have been responsible for increased development of electrical systems capable of operating at much higher frequencies than heretofore. Inasmuch as, at least for the present, there is still a predominance of electrical systems, for such systems to be competitive, they must operate at the higher frequencies of which optical systems are capable.
In an electrical communication system, it is sometimes advantageous to transmit information (video, audio, data) in the form of balanced signals over a pair of wires (hereinafter "wire-pair") rather than a single wire, wherein the transmitted signal comprises the voltage difference between the wires without regard to the absolute voltages present. Each wire in a wire-pair is capable of picking up electrical noise from sources such as lightning, automobile spark plugs and radio stations to name but a few. Balance is affected by impedance symmetry in a wire-pair as between its individual conductors and ground. When the impedance to ground for one conductor is different than the impedance to ground for the other conductor, then common mode (longitudinal) signals are undesirably converted to differential mode (transverse) signals and vice versa. Additionally, return loss is a reflection of the incoming signal, and it occurs when the terminating impedance does not match the source impedance. Of greater concern, however, is the electrical noise that is picked up from nearby wires that may extend in the same general direction for long distances. This is referred to as crosstalk, and so long as the same noise signal is added to each wire in the wire-pair, then the voltage difference between the wires will remain the same. In all of the above situations, undesirable signals are present on the electrical conductors that can interfere with the information signal. Existing crosstalk compensation schemes in connectors for adjacent pairs of conductors are designed to compensate for differential crosstalk on an idle pair induced, i.e., coupled, from a nearby driven pair. However, most such schemes do not provide for compensation for the differential-to-common mode crosstalk between the driven pair and the idle pair. In the absence of compensation for this latter form of crosstalk, an unbalanced signal is induced in the adjacent pair. Thus, to achieve balance, it is desirable to compensate not only for differential crosstalk caused by a differential input signal, but, also, to compensate for common mode crosstalk caused by a differential input signal and differential mode crosstalk caused by a common mode signal. In U.S. Pat. No. 5,967,853 of Hashim, the disclosure of which is incorporated herein by reference, there is shown a compensation arrangement using capacitors between different pairs of conductors which offset both differential-to-differential crosstalk coupling as well as differential-to-common-mode coupling. The capacitors generally are designed within a printed wiring board (PWB) connected to the connector, and are carefully chosen as to value to produce the desired amount of compensation (or coupling) between discrete pairs. In any such compensation arrangement, design techniques require good judgement and are applicable only to achieve a certain level of balance performance for the specific parameters of the signal transmission.
In U.S. Pat. No. 5,186,647 to Denkmann et al. and U.S. Pat. No. 5,997,358 of Adriaenssens et al., the disclosures of which are incorporated herein by reference, there are shown connectors wherein compensating crosstalk is introduced by establishing stages wherein predetermined magnitudes and phases of compensating crosstalk are generated. The stages are created by cross-overs of certain conductors within the connector or by appropriately placed capacitors. Both patents disclose differential crosstalk compensation but do not address the differential-to-common mode crosstalk, as is done in the Hashim patent.